When power is applied from a d.c. or a.c. power supply to a small gap formed between a tool electrode of an electric discharge machining apparatus and a conductive workpiece, simply known as a "machining gap" or "gap", the resistance of dielectric fluid across the gap is reduced. Then, when the insulation properties of the dielectric fluid are broken down, an electric discharge is generated and machining "on-time" begins. During a controlled on-time, a discharge current flows through the gap resulting in vaporization or melting of the workpiece material. When the on-time is completed, application of power is suspended resulting in controlled "off-time", in order to restore the insulation properties of the dielectric fluid. A decrease in the on-time, in other words a reduction in energy for one electric discharge, is known to contribute to an improvement in surface roughness.
U.S. Pat. No. 5,149,931 discloses an electric discharge machining method for causing the surface roughness of a workpiece to be reduced to 1 mRmax or less by applying high frequency a.c. voltage from an a.c. power source to the gap.
This patent also describes that changing the polarity of the voltage applied to the machining gap for each electric discharge and distributing the position at which electric discharge occurs contributes to a good quality machined surface.
FIG. 3 illustrates a power supply device for use in an electric discharge machining apparatus that generates a.c. voltage pulses from a d.c. power source. The power supply includes a d.c. power source 2 for outputting a d.c. voltage E, an output capacitor C and a bridge circuit 3. These electrical components are usually housed in a cabinet which also includes a controller which controls the on-time and off-time, and which is positioned at a distance away from the workpiece 61 and the wire electrode 62. In the illustrated embodiment, the bridge circuit 3 is connected through a low capacitance cable 7 to the workpiece 61 and the wire electrode 62. As shown in the drawing, the bridge circuit 3 comprises switching transistors 31, 32, 33 and 34 connected in series so as to form four nodes 3A, 3B, 3C and 3D. One pair of diagonally opposite nodes 3A and 3B are respectively connected to positive and negative terminals of the d.c. power source 2. The other pair of nodes, 3C and 3D, are respectively connected to the workpiece 61 and the wire electrode 62. A current limiting resistor 35 is connected between the nodes 3A and 3C, and a current limiting resistor 36 is connected between nodes 3B and 3D. A controller 4 generates a gate control pulse signal PA for controlling the on/off switching operation of one pair of switching transistors 31 and 34, and a gate control pulse signal PB for controlling the on/off switching operation of the other pair of switching transistors 32 and 33. The controller 4 generates the pulse signals PA and PB to alternately switch on and off the pair of switching transistors 31 and 34 and the pair of switching transistors 32 and 33. As a result, an a.c. pulse voltage PV inverted in polarity at the same frequency as the pulse signal PA across the nodes 3C and 3D of the bridge circuit 3 is applied as a voltage V to a gap G, formed between the workpiece 61 and the wire electrode 62, to machine the workpiece 61.
Since the surface roughness of the workpiece 61 becomes smaller as the frequency of the a.c. pulse voltage VP increases, MOSFETs that have a high operating speed are often used as the switching transistors 31, 32, 33 and 34. In order to operate these MOSFETs at a high frequency, i.e., on the order of a few MHz, a MOSFET of at least 50 W is necessary. Thus is true even in the lowest frequency case, taking into consideration the effects of stray capacitance and distributed inductance, etc. of the circuit from the nodes 3C and 3D to the gap G. Also, the rated voltage of a MOSFET for supplying a voltage of 50-100V necessary to generate electric discharge at the gap G is preferably at least 200V. Because of these requirements, a MOSFET for a bridge circuit for generating a high frequency a.c. voltage pulse from a d.c. power source has an input capacitance of 700-1600 pF. The relationship between the input capacitance Cin and power loss Pd occurring as a result of driving the MOSFET is given below where VGS is a voltage across the gate and source of the MOSFET, f is the frequency of the a.c. voltage pulses: EQU Pd=Cin.multidot.f.multidot.VGS.sup.2 (1)
Accordingly, if, for example, Cin=700 pF, VGS=20 V and f=5 MHz, power loss will be 1.4 W. The rated value of a commercial DIP (Dual In-line Package) is less than 1 W. This means that if the drive loss is 1.4 W, the DIP will need to be fitted with a heat dissipation device, such as fins. However, the surface area occupied by such cooling fins on the printed substrate of the DIP is relatively large and as a result the physical distance between the drive circuit and the MOSFET is increased. There is also undesirable series resonance caused by the inductance between the drive circuit and the MOSFET and the input capacitance (Cin) of the MOSFET.